Visible light responsive photocatalysts are photocatalysts that can utilize visible light, contained in a large amount in sunlight. The visible light responsive photocatalysts are expected to be applied to photodecomposition of organic substances and hydrogen production by photolysis of water. In particular, photocatalysts for splitting water have drawn attention as photocatalysts for producing hydrogen by utilizing the renewable energy. As a result, there is an ever-increasing demand for highly active photocatalysts for splitting water.
Rhodium-doped strontium titanate (Rh—SrTiO3) as a visible light responsive photocatalyst for splitting water is known to have a very high capability of generating hydrogen by photolysis of water. A Z-scheme system of a combination of Rh—SrTiO3 with a photocatalyst for oxygen generation is known to provide a high energy conversion efficiency in a water splitting reaction (Japanese Patent Application Laid-Open No. 2004-008963 (PTL 1) and Sasaki et al., J. Phys. Chem. C 17536-17542 pages, 2009 (NPTL 1)).
The above Rh—SrTiO3 has hitherto been prepared by a solid-phase reaction method or a hydrothermal synthesis method. In these methods, it is known that a treatment is carried out that includes firing a material at about 1000° C. to render the material highly crystalline. Rh—SrTiO3 particles thus obtained are known to have a primary particle diameter of approximately several hundred nanometers to a few micrometers and have a high hydrogen generation capability under visible light irradiation. Rh—SrTiO3 particles are required to be in a microcrystalline form to have an increased specific surface area so that Rh—SrTiO3 particles are highly activated.